1. Field of the Invention
The present invention relates to a diesel engine equipped with a mechanically driven charger.
2. Description of the Related Art
Two of the great advantages of diesel engines are that, one, the cost of fuel used for diesel engines is cheaper than that of gasoline, in some countries such as Japan, and two, that the fuel consumption of diesel engines is lower than that of gasoline engines. Consequently, in an effort to enhance the advantages of diesel engines, attempts have been made to reduce the fuel consumption as much as possible. In a diesel engine having a swirl chamber or a prechamber, the fuel consumption and the output power after completion of the warm-up are changed in accordance with the geometrical compression ratio .epsilon. (hereinafter referred to as compression ratio) as illustrated in FIG. 6, and in such a diesel engine, the lowest fuel consumption can be obtained when the compression ratio .epsilon. is about 16:1 through 18:1. Consequently, in such a diesel engine, preferably the combustion chamber has a compression ratio .epsilon. of about 16:1 through 18:1.
However, in this diesel engine, when the piston is close to the top dead center of the compression stroke, if the temperature of air under pressure in the combustion chamber does not increase beyond the ignition temperature of fuel injected from a fuel injector, it is impossible to ignite and burn the fuel. In a diesel engine having the compression ratio of about 16:1 through 18:1, the temperature of air in the combustion chamber increases beyond the ignition temperature of the fuel after the engine is warmed-up. However, in a particular engine operating state such as a cold start of the engine, the temperature of air in the combustion chamber does not increase beyond the ignition temperature of the fuel, and thus it is impossible to operate the engine. In addition, when the amount of fuel injected from the fuel injector is small, and thus the heat produced in the combustion chamber is small, such as during idling or in an extremely low load operating state, although the temperature of air in the combustion chamber may increase beyond the ignition temperature of the fuel, the increase in the temperature of the fuel is insufficient, causing an incomplete combustion, misfiring, and the production of white smoke. Furthermore, when the engine is operated under a low atmospheric pressure, for example, at a high altitude, the range of the engine operating state in which the temperature of air in the combustion chamber does not increase beyond the ignition temperature of the fuel, is widened. Therefore, in a conventional diesel engine, the engine normally has a compression ratio of about 20:1 through 23:1, so that the temperature of air in the combustion chamber can increase beyond the ignition temperature of the fuel even when a cold start is carried out or when the engine is operated at a high altitude. However, if the engine has a compression ratio of about 20:1 through 23:1, the fuel consumption after warm-up increases, as illustrated in FIG. 6, eliminating one of the great advantages of the diesel engine.
In order to increase the output power of an engine, a diesel equipped with a turbocharger is known. However, since the turbocharger is operated by the energy of the exhaust gas, when the engine is operating at a low speed or under a light load, the rotating speed of the turbocharger can not be sufficiently increased due to a lack of energy of the exhaust gas, and thus a turbocharging operation is not sufficiently carried out. Consequently, even if the diesel engine is equipped with a turbocharger, if the engine has a compression ratio of about 16:1 through 18:1, the temperature of air in the combustion chamber can not increase beyond the ignition temperature of the fuel at the time of cold start of the engine. Consequently, in a diesel engine equipped with a turbocharger, the engine engine must have a compression ratio of about 20:1 through 23:1.
In addition, another diesel engine equipped with a charger mechanically driven by the engine is known. In this diesel engine, since the charger is driven when the engine is started, the temperature of air in the combustion chamber is increased at the start of the engine because of the additional charged air to the engine. Consequently, in this diesel engine, even if the engine has a compression ratio of about 16:1 through 18:1, the temperature of air in the combustion chamber increases beyond the ignition temperature of the fuel. Therefore, in this engine, since it is possible for the engine to have a compression ratio of 16:1 through 18:1, it is possible to improve the fuel consumption. However, in this engine, the charger is continuously driven by the engine, and thus the charging operation is carried out when the engine is operating under a middle load in which the charging operation is not necessary. Consequently, when the engine is operating under a middle load, the compressing force to be exerted by the piston increases, and at the same time, a loss of output power of the engine occurs due to the necessity for driving the charger. Therefore, in this diesel engine, a problem occurs in that the fuel consumption is increased.
A further diesel engine is known in which the turbocharger and the mechanically driven charger are arranged in series in the intake passage of the engine. In this engine, a bypass passage bypassing the mechanically driven charger is provided (Japanese Unexamined Utility Model Publication No. 59-67537). When the rotating speed of the engine is higher than a predetermined speed, the operation of the mechanically driven charger is stopped, and when the rotating speed of the engine is lower than a predetermined speed, the mechanically driven charger is operated. In this diesel engine, when the engine is operating at a low speed at which the turbocharger does not work, since the charging operation is carried out by the mechanically driven charger, it is possible to increase the output power of the engine even when the engine is operating at a low speed. In addition, when the rotating speed of the engine increases, since the operation of the mechanically driven charger is stopped, it is possible to improve the fuel consumption.
In this engine, the mechanically driven charger is operated when the engine is operating at a low speed as mentioned above, and the object of this is to improve the output power of the engine when the engine is operating at a low speed. Therefore, even if the mechanically driven charger is controlled so that it is operated when the engine is operating at a low speed, it is unclear whether or not the mechanically driven charger is operating at the time of cranking of the engine, that is, at a start of the engine, at which the engine does not produce output power. In this engine, if the mechanically drive charger is not operated when the engine is started, it is impossible to lower the compression ratio, and therefore, it is impossible to obtain a good fuel consumption.